1. Field of the Invention
The present invention relates to a failure diagnosis apparatus for diagnosing a failure in an evaporative fuel processing system which temporarily stores evaporative fuel generated in a fuel tank and supplies the stored evaporative fuel to an internal combustion engine.
2. Related Art
If a leak occurs in an evaporative fuel processing system which temporarily stores evaporative fuel generated in a fuel tank and supplies the stored evaporative fuel to an internal combustion engine, the evaporative fuel is released into the atmosphere. Accordingly, various leak determination methods have been proposed. For example, Japanese Patent Laid-open No. Hei 11-336626 discloses a method for determining a leak after stoppage of the engine rather than during operation of the engine.
According to this conventional method, a change in a pressure difference between a pressure in an evaporative fuel processing system and atmospheric pressure is determined after stoppage of the engine. Leak determination is performed according to an amount of change in the determined pressure difference.
In this conventional method, leak determination is performed according to the amount of change in the pressure in the evaporative fuel processing system due to a change in the temperature in a fuel tank after stoppage of the engine. Accordingly, when a temperature rise in the fuel tank is insufficient, as in the case of stopping the engine immediately after starting of the engine, the temperature change after stoppage of the engine is small and the pressure change is accordingly small. In such case, there is a high possibility of improper determination.
It is accordingly an object of the present invention to provide a failure diagnosis apparatus for an evaporative fuel processing system which can prevent improper determination and improve determination accuracy when a leak determination of the evaporative fuel processing system is performed after stoppage of the engine.
The present invention provides a failure diagnosis apparatus for diagnosing a failure in an evaporative fuel processing system. The evaporative fuel processing system has a fuel tank, a canister containing an adsorbent for adsorbing evaporative fuel generated in the fuel tank, an air passage connected to the canister and communicating with the atmosphere, a first passage for connecting the canister and the fuel tank, a second passage for connecting the canister and an intake system of an internal combustion engine, a vent shut valve for opening and closing the air passage, and a purge control valve provided in the second passage. The failure diagnosis apparatus includes pressure detecting means, engine stoppage detecting means, determining means, gas layer temperature detecting means, ambient temperature detecting means, and inhibiting means. The pressure detecting means detects a pressure in the evaporative fuel processing system. The engine stoppage detecting means detects stoppage of the engine. The determining means closes the purge control valve and the vent shut valve when stoppage of the engine is detected by the engine stoppage detecting means and determines whether there is a leak in the evaporative fuel processing system according to the pressure detected by the pressure detecting means during a predetermined determination time period after closing the purge control valve and the vent shut valve. The gas layer temperature detecting means detects a gas layer temperature in the fuel tank, and the ambient temperature detecting means detects an ambient temperature. The inhibiting means inhibits the determination by the determining means when the difference between the gas layer temperature and the ambient temperature detected respectively by the gas layer temperature detecting means and the ambient temperature detecting means upon stoppage of the engine is less than or equal to a predetermined threshold value.
With this configuration, when the stoppage of the engine is detected, the purge control valve and the vent shut valve are closed and the leak determination of the evaporative fuel processing system is performed according to the pressure detected by the pressure detecting means during the predetermined determination time period after closing the purge control valve and the vent shut valve. When the difference between the gas layer temperature and the ambient temperature detected upon stoppage of the engine is less than or equal to the predetermined threshold value, the leak determination is inhibited. Accordingly, when the gas layer temperature in the fuel tank is not much higher than the ambient temperature, that is, when the engine is stopped immediately after starting, for example, the leak determination is inhibited to thereby prevent improper determination.
Preferably, the inhibiting means includes abnormality detecting means for detecting an abnormality in at least one of the pressure detecting means and the vent shut valve, and inhibits the determination by the determining means when an abnormality is detected by the abnormality detecting means.
With this configuration, the improper determination due to the abnormality in the pressure detecting means or the vent shut valve can be prevented.
Preferably, the determining means executes a first open-to-atmosphere process for maintaining the vent shut valve in an open condition immediately after detection of the stoppage of the engine to make the pressure in the evaporative fuel processing system equal to the atmospheric pressure, and further executes a first monitoring process for closing the vent shut valve after the first open-to-atmosphere process ends to determine a change in the pressure detected by the pressure detecting means after closing the vent shut valve. Then, the determining means determines that the evaporative fuel processing system is normal when the pressure detected by the pressure detecting means becomes greater than a first predetermined pressure during execution of the first monitoring process.
Preferably, the determining means executes a second open-to-atmosphere process for opening the vent shut valve after the first monitoring process ends to make the pressure in the evaporative fuel processing system equal to atmospheric pressure, and further executes a second monitoring process for closing the vent shut valve after the second open-to-atmosphere process ends to monitor a change in the pressure detected by the pressure detecting means after closing the vent shut valve. Then, the determining means determines that the evaporative fuel processing system is normal when the pressure detected by the pressure detecting means becomes less than a second predetermined pressure during execution of the second monitoring process.
Preferably, the determining means stores a maximum value of the pressure detected by the pressure detecting means during execution of the first monitoring process, and further stores a minimum value of the pressure detected by the pressure detecting means during execution of the second monitoring process. Then, the determining means determines that there exists a leak in the evaporative fuel processing system, when the difference between the stored maximum value of the pressure detected by the pressure detecting means and the stored minimum value of the pressure detected by the pressure detecting means is less than or equal to a predetermined pressure difference.